1. Field of the Invention
The invention is directed to a process for phenol rectification at a sufficiently high temperature level to recover the heat of condensation for steam generation while reducing undesirable impurities in product phenol.
2. Description of Related Art
This invention relates to a method to prepare phenol from the products of decomposition (cleavage) of cumene hydroperoxide. Cumene hydroperoxide is decomposed to yield various product components including phenol and acetone under acidic catalysis.
The decomposition product is neutralized, typically in an alkaline ion exchanger, before being separated into product phenol, byproducts and unreacted cumene for recycle. The ion exchange beds are operated in cycles, between regenerations, and the neutralization operation is controlled to hold the pH of the decomposition mass in the range of 2 to 4. A wide variety of byproduct organic acids in considerable amount remain in the neutralized decomposition crude product. Common organic acids in the neutralized decomposition mass include benzoic acid, formic acid, acetic acid, maleic acid, lactic acid, propionic acid, isobutyric acid, oxalic acid, succinic acid, valeric acid, and isovaleric acid. The total acidity, under normal operating conditions, is in the range of 50-500 ppm, but typically falls between 150 and 300 ppm. The sources of the acidity are likely to be the oxidized fractions of cumene molecules and impurities in the feed stocks.
The decomposition mass is separated sequentially into acetone, cumene, alpha methyl styrene (AMS), phenol and high boiling residues. Phenol, acetone and AMS are further purified into end products. Recovered cumene is caustic washed to rid it of acidity before being recycled to oxidation. The separation of the crude decomposition mass is carried out in a series of fractionating distillation columns in commercial operation. Thus substantial portions of acetone, cumene and AMS are removed by distillation, with a resulting crude phenol mass which can be rectified to obtain product-grade phenol. Significant amounts of acid containing compounds evade front end distillation and enter the phenol purification column with the crude phenol mass, where acids are released under vacuum and elevated temperature conditions.
Purification of the crude phenol mass is carried out in a vacuum distillation column usually equipped with from 30 to 40 vapor-liquid counter current contact trays or equivalent effective height of mass transfer packed beds. The vacuum level in the distillation column overhead ranges from 50 to 300 mmHg. The overhead vapors are withdrawn from the column and totally condensed. To reduce light impurities in the product phenol, a small fraction of the condensed overhead is removed from the phenol purification column and recycled upstream. This recycle stream can vary from 3 to 15 weight percent of the total overhead condensate. The balance of the overhead condensate is returned to the column to provide reflux. Product phenol is withdrawn from a few trays below the overhead.
The acidity in the top of the phenol purification column acts as catalyst to generate impurities affecting phenol quality. On the merchant market water light transmission, a measure of the amount of water insoluble organic compounds, is the prevailing specification for phenol sales. The acid catalyzed products have been proven to be responsible for phenol failing the water light test. In the use of phenol for the manufacture of cyclohexanone by hydrogenation, acids have been found to be poisonous to the palladium catalyst.
Thus, it is desirable to develop a process for achieving sales grade phenol in an economic manner. U.S. patent application Ser. No. 650,253, filed Feb. 4, 1991 discloses a method wherein injection of water into the overhead of the phenol distillation column has been successful in improving the water light transmission of the phenol product. Water injection has the apparent effect of reducing acidity and light impurities in phenol and mitigating any acid catalyzed reactions. However, water injection lowers the column overhead temperature substantially. This prevents the use of the overhead condenser to generate low pressure steam. At the overhead pressure of 150 mmHg, a typical condition for the phenol purification column, the condensing phenol temperature is around 130.degree. C., suitable for generating 10 psig. steam for a variety of process and utility uses. With water injection in sufficient amount to attain a water concentration level necessary for product passing the water light test, the corresponding condensing temperature falls below 60.degree. C., ruling out steam generation. The value of steam generation is considerable. Steam generation can recover substantially all the energy input to the distillation operation, although at a lower pressure level.